1. Field of Invention
The present invention relates generally to gas monitoring techniques and apparatus. More specifically, the present invention relates to a method and apparatus for monitoring the respiratory gases of spontaneously breathing patients who are receiving supplemental oxygen.
2. Description of the Related Art
Supplemental oxygen is widely used for the long-term treatment of chronically ill patients suffering from various respiratory diseases such as Chronic Obstructive Pulmonary Disease (COPD) and emphysema. Additionally, in emergency situations, supplemental oxygen is administered on a short-term basis to relieve acute symptoms, such as shortness of breath and lowered oxygen saturation. Supplemental oxygen is also commonly administered throughout the hospital setting, such as in the operating room during surgery and post-op, and in the intensive care units to critically ill patients.
Conventional practices for administering supplemental oxygen to a patient include nasal cannulae and face masks. The nasal cannula consists of tubing with a pair of stubs that are situated within the nostrils of the patient and through which oxygen flows. The nasal cannula provides more freedom of movement for the patient than other methods but drawbacks of using the nasal cannula are well known and include unknown delivered FIO2 (fraction of inspired oxygen), irritation of the nose and easy dislodgment of the cannula from the patient's nostrils. Moreover, cannulae including a gas monitoring capability exhibit a recognized inability to detect both oral and nasal gas exchange as well as a tendency to dilute the measured gas.
A nasal cannula design that purportedly overcomes the nasal irritation problem and reduces the potential for easy dislodgment from the nostrils is disclosed in U.S. Pat. No. 6,247,470 (hereinafter “the '470 Patent”), issued to Ketchedjian on Jun. 19, 2001. The '470 patent uses a flexible and adjustable lever arm placed with its free end adjacent the patient's mouth to direct oxygen toward the user's oral and nasal cavities and to intake exhaled CO2 for monitoring purposes. Thus, the device is similar in configuration to a telephone headset for hands-free speaking.
While this device is not subject to nasal dislodgment, with its unwieldy fixation scheme it is subject to easy spatial relocation, potentially dramatically reducing the efficiency of the oxygen delivery. Further, a clinical study published by Bazuaye (Bazuaye E A et al. Variability of inspired oxygen concentration with nasal cannulas. Thorax. 1992 Dec; 47(12): 1086) concludes that “‘Typical’ values of FIO2 quoted with nasal cannulas can mislead . . . confirm(ing) that this mode of oxygen delivery is unsatisfactory if precise control of inspired oxygen is desired.” Thus, while the apparatus of the '470 patent provides a monitoring capability combined with oxygen delivery, its inefficient delivery in comparison to face masks and the unenclosed gas sampling location relatively distant from oral and nasal cavities makes this apparatus of questionable clinical utility.
Oxygen delivery cannulae incorporating a sidestream CO2 monitoring capability such as the NAZORCAP™ sampler offered by NAZORCAP Medical, Inc. of West Mifflin, Pa. and further described in U.S. Pat. No. 5,046,491, issued to Derrick on Sep. 10, 1991, also have a number of drawbacks. For instance, such oxygen delivery cannulae exhibit problems with moisture and mucous plugs, switching back and forth between oral and nasal breathing, and dilution of CO2 readings with administered O2.
Oxygen masks, which are simple, inexpensive to use, and not subject to easy dislodgment, have also been employed to reliably administer oxygen levels of 40–60% O2 to the patient. Oxygen mask designs vary based upon intended use of the particular mask. Oxygen masks include a body that is sized to seat over the nose and mouth of the patient on whom the mask is to be placed. With conventional mask designs, oxygen is introduced through an oxygen inlet, and expiratory gases are vented from the mask through apertures.
Disadvantages of conventional oxygen mask delivery systems include wasted oxygen because the oxygen flow continues unabated directly into the mask during exhalation and could be more efficiently delivered to the patient. Depending upon the mask design, the patient may not tolerate a mask for more than short periods of time. Also, no quantitative monitoring of the end-tidal carbon dioxide is performed. Such monitoring would allow for the diagnosis of hypercapnia, which indicates inadequate oxygen delivery and the need for a more aggressive treatment strategy.
While recent designs have addressed these disadvantages of wasted oxygen and gas measurement separately, an apparatus providing for better titration of oxygen to the patient would be advantageous.
One design that purportedly provides enhanced efficiency of oxygen use was recently disclosed in U.S. Pat. No. 6,192,884, issued to Vann et al. on Feb. 27, 2001. The enhanced efficiency is described as relating to the administration of an oxygen bolus at the beginning of each inhalation by a patient.
A mask design with a qualitative calorimetric sensor that purportedly senses the presence or absence of carbon dioxide and is integral and in intimate contact with the mask housing was disclosed in U.S. Pat. No. 5,857,460 (hereinafter “the '460 Patent”), issued to Popitz et al. on Jan. 12, 1999. However, the device of the '460 Patent is not positioned in the respiratory gas stream and, as such, only assesses the presence or absence of carbon dioxide and provides only information that is, at best, qualitative.
Thus, recent designs have attempted to separately address the problems of wasted oxygen and gas measurement. However, a design which addresses them in combination to promote better titration of oxygen to the patient would be desirable.
A type of patient sedation termed “conscious sedation” has been recognized as advantageous for certain types of surgical and diagnostic procedures and is becoming ever more prevalent. This sedation type induces an altered state of consciousness that can minimize pain and discomfort through the use of pain relievers and sedatives. Patients who receive conscious sedation are usually able to speak out and respond to verbal cues throughout a procedure, enabling them to communicate any discomfort they experience to the health care provider. A brief period of amnesia may subsequently erase any memory of the procedure. Further, conscious sedation allows patients to recover quickly and resume normal daily activities in a short period of time.
Conscious sedation provides a safe and effective option for patients undergoing minor surgical or diagnostic procedures. The number and types of procedures that can be performed using conscious sedation have increased significantly as a result of new technology and state of the art pharmaceuticals. Exemplary procedures with which conscious sedation is useful include breast biopsy, vasectomy, minor foot surgery, minor bone fracture repair, plastic or reconstructive surgery, dental prosthetic or reconstructive surgery, and endoscopy, such as for diagnostic studies and treatment of stomach, colon and bladder.
Non-intubated gas exchange monitoring of the sedated, conscious patient is necessary because patients can slip into a deep sleep. However, conventional techniques as described above afford inadequate monitoring capability to the clinician.